Deepwater reservoirs containing hydrocarbons often have complex internal stratigraphy that needs to be understood prior to field development. For this reason high-resolution target-oriented seismic surveys are sometimes acquired prior to finalizing deepwater oil and gas-field development plans. However, as acquisition technology and imaging algorithms improve, the ultimate resolution may become limited by the low-pass filtering effects of the earth from seismic attenuation. Seismic attenuation is the frequency-dependent reduction in amplitude or energy in a seismic wave as the wave passes farther away from a source due to microscopic frictional forces and scattering from thin layers. It is often described in terms of a seismic quality factor, Q. Seismic attenuation is affected by fluid saturations, clay content and thin-layering. There is a danger that if sufficient attenuation occurs, the additional uplift provided by expensive increased spatial sampling of seismic data will be minimal.
Seismic quality factor, Q, estimates are valuable for seismic imaging, processing, and reservoir characterization applications. Examples of such applications include amplitude and phase compensation, wavelet processing, acquisition design, and lithology/fluid identification. Furthermore, unlike many other seismic attributes, seismic attenuation may be directly related to permeability (via a squirt flow mechanism, for example). By combining rock physics models with recent advances in time-frequency analysis, links can be made between estimated seismic quality factors and key reservoir parameters.
The classic method for estimating the effective attenuation between two seismic waveforms is the spectral ratio method. In this approach, the log of the ratio between two amplitude spectra is computed as function of frequency, and the slope of a best-fit line is related to 1/Q. White, R. E., The accuracy of estimating Q from seismic data: Geophysics, 57, no. 11, 1508-1511, (1992) describes fundamental limitations in resolution with this method. White concludes that although the errors may be reasonable for VSP (Vertical Seismic Profiles) and well-tied surface seismic data, the variance of errors when Q is estimated from surface seismic data will never be less than about 50%.
Grossman et al., J., Lamoureux, M., Aggarwala, R., and Margrave, G., A robust algorithm for constant-Q wavelet estimation using Gabor analysis: 72nd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, (2002) and Margrave, G., Henley, D., Lamoureux, M., Iliescu, V., and Grossman, J., Gabor deconvolution revisited: 73rd Ann. Internat. Mtg., Soc. Expl. Geophys., Expanded Abstracts, 714-717 (2003), estimate Q with a technique based on the Gabor-transform. While seeking a global solution that is consistent with a single effective waveform, their technique has the disadvantages that it only works for a single, depth-independent Q value. Furthermore, this method is sensitive to the absolute scaling of the waveforms. Quan, Y., and Harris, J. M., Seismic attenuation tomography using the frequency shift method: Geophysics, 62, no. 3, 895-905 (1997), developed a tomographic technique that uses the shift in central frequency of seismic waveforms to infer interval attenuation. While their approach has the advantage that it is insensitive to absolute scaling, the approach relies on simplifying assumptions about the spectra of the input waveforms. This is a disadvantage because seismic waveforms are never simple, and the effects of attenuation are most significant and visible at the high frequencies.
Therefore, there is a need for a method to determine estimates of seismic quality factors, Q, which are insensitive to absolute scaling and utilize the entire bandwidth of the seismic signal. The present invention addresses this need.